Near field communication, more commonly referred to by the person skilled in the art as “NFC,” is a wireless connectivity technology which that a communication over a short distance, for example 10 cm, between electronic devices, such as, for example, contactless chip cards or mobile telephones in card-emulation mode, and readers.
NFC technology is particularly suitable for connecting any type of user device and allows fast and simple communications.
A contactless object is an object capable of exchanging information via an antenna with another contactless object, for example a reader, according to a contactless communication protocol.
An NFC object, which is a contactless object, is an object compatible with NFC technology.
NFC technology is an open technology platform standardized in the ISO/IEC 18092 and ISO/IEC 21481 standards, but incorporating many existing standards, such as, for example, the type A and type B protocols defined in the ISO-14443 standard, which may be communication protocols usable in NFC technology.
Apart from its conventional telephony function, a mobile cellular telephone can be used (if it is equipped with specific means) to exchange information with another contactless device, for example a contactless reader, by using a contactless communication protocol usable in NFC technology.
This allows information to be exchanged between the contactless reader and secure elements located in the mobile telephone. Many applications are thus possible, such as mobile ticketing in public transport (the mobile telephone acts as a transport ticket) or mobile payment (the mobile telephone acts as payment card).
During a transmission of information between a reader and an object in card-emulation mode or tag-emulation mode, the reader generates via its antenna a magnetic field which, in the standards conventionally used, is generally a 13.56 MHz sine wave. The strength of the magnetic field is between 0.5 and 7.5 amperes/metre RMS (Root Mean Square).
Two modes are then possible, a passive mode and an active mode.
In the passive mode, only the reader generates the magnetic field and the object, in card-emulation mode or tag-emulation mode, is then passive and always acts as the target.
More precisely, the antenna of the tag-emulating or card-emulating object modulates the field generated by the reader.
This modulation is carried out by modifying the load connected to the terminals of the antenna of the object.
By modifying the load on the terminals of the antenna of the object, the output impedance of the antenna of the reader changes due to the magnetic coupling between the two antennas. This results in a change in the amplitudes and/or phases of the voltages and currents present in the antennas of the reader and the object.
And, in this way, the information to be transmitted from the object to the reader is transmitted by way of load modulation on the antenna currents of the reader.
The load variation performed during the load modulation manifests itself as an amplitude modulation and/or phase modulation of the signal (voltage or current) in the antenna of the reader. A copy of the antenna current is generated and injected into the receiving chain of the reader where this current is demodulated and processed in such a way as to extract the transmitted information.
In the active mode of operation, the reader and the object in card-emulation mode both generate an electromagnetic field. This mode of operation is generally used when the object is provided with its own power source, for example a battery, as in the case of a mobile cellular telephone which is then in card-emulation mode.
Each of the NFC devices transmits the data by using a modulation scheme, typically an ASK (“Amplitude Shift Keying”) scheme.
Here also, the modulation manifests itself as a load modification and this is then referred to as communication by way of active load modulation.
Compared with a passive communication mode, greater operating distances are obtained which may extend to 20 cm according to the protocol that is used.
Furthermore, the use of active load modulation allows the use of smaller antennas.
However, this type of communication using active load modulation poses other problems.
In fact, during the active communication periods of the device in card-emulation mode, the electromagnetic field of the reader is not directly observable. This may result in a non-synchronous response of the object in card-emulation mode and therefore in a signal received by the reader having a phase shift, particularly during long periods of transmission by the device in card-emulation mode.
Thus, if two independent devices are considered, i.e. the reader and an object in card-emulation mode, capable of contactless communication by way of active load modulation, a need consequently exists to minimize or even eliminate this phase shift.